After he and two friends also had their fingerprints and photos taken, all three were also denied entry.

A US Customs Officer checks the identification papers of a driver entering the US from Canada (file picture).Reuters

"They told me I was being denied entry for administrative reasons. According to the agent, my travelling to the United States for the purpose of protesting didn't constitute a valid reason to cross," Decunha said.

"It's concerning to see that at border crossings you're being screened for what your political beliefs are. It felt like if we had been pro-Trump, we would have absolutely been allowed entry."

US Customs and Border Protection said it could not discuss individual cases, citing privacy reasons.

"We recognize that there is an important balance to strike between securing our borders while facilitating the high volume of legitimate trade and travel that crosses our borders every day, and we strive to achieve that balance and show the world that the United States is a welcoming nation," it said in an email to the Guardian.

The agency added more than one million individuals are admitted to the US on a daily basis. An average of 600 people a day – less than a tenth of 1% of those admitted – are denied entry for a varied list of reasons, such as national security.